When banknotes (may also be referred to as “notes” herein) pass through a high-speed transport system along a banknote transport path, the banknotes may tend to run through at non-zero angles relative to the transport centerline. As a result, the images of such banknotes taken by an optical sensor positioned in proximity to the transport system will be skewed. The skew angle of the banknote must be determined (or at least estimated within a specified criteria) in order for many types of banknote processes to perform correctly. Since this skew angle can be random from banknote to banknote, a skew angle determination method must estimate the angle based solely on information collected from the current banknote instead of relying on information from prior banknotes or from a calibration procedure. Further, some banknotes may be torn and lack a regular rectangular border. Such banknote fragments cause significant problems for geometric-based algorithms. Yet still further, banknotes tend to have more printed features that are vertically aligned (i.e., aligned along the lengthwise axis of a banknote (for example, the axis aligned vertically with the page of the example note shown in FIG. 3A)) than typical text documents that mostly or only contain lines of text (which are horizontally aligned). Therefore, it is much more difficult to determine the skew angle of a banknote (or any document that contains a larger percentage of printed images relative to printed text) than a skew angle of a typical text-dominated document.
U.S. patent application Ser. No. 12/904,908, which is hereby incorporated by reference herein, disclosed a document sensor system for creating images of scanned documents (e.g., banknotes) in anticipation of performing various processes on such images. The present application provides details of processes for determining or estimating the skew angle of a document, such as those that pass through the document sensor system described in Ser. No. 12/904,908. Various aspects of this document sensor system are repeated herein in order to assist in describing how the present invention may operate in a document sensor system. Note, however, that embodiments of the present invention are not limited to implementations within such a document sensor system.
As previously noted, determining the skew angle of a banknote image is an important prerequisite for many banknote processes. Skew correction requires the determination of a skew angle and the modification of a document image representation based on the skew angle. Embodiments of the present invention estimate the skew angle by examining patterns found within a two-dimensional Fast Fourier Transform (“FFT”) of a subset of the banknote image. This technique exploits the internal structure of currency (banknote) designs to allow fast and accurate skew angle estimation for arbitrary banknote fragments that are problematic for existing solutions. The terms “skew angle determination,” “skew angle estimation,” and “skew angle estimate” are used interchangeably herein. When the skew angle is determined, its value is an estimate that is a function of the parameters used in the systems and methods described herein.